if a knife won't fit then you will need to open it up in stages. you can get stainless steel shim stock that may be useful. However, unless this is a REALLY special coin, the price of the various sizes of shim stock you'll need will likely cost more than the value of the coin. Otherwise, you could heat the piece of coin very hot to make it softer if it is a pure silver coin. If it's not a real silver coin, I wouldn't bother. A new high quality quarter would cost very little at a decent magic store. A Johnson coin is going to be way better than most of the junk coins sold by other vendors. A cheapo coin can be had for under ten bucks that will do the job.

A machinist or gunsmith could could open it up but they'll charge you at least a hundred dollars just to start working on it. Setup time would be at least ten times the value of the coin.